When building large concrete structures e.g. walls, floors, columns etc. in buildings, grids of reinforcing bars must typically be laid out and bound together on site before the concrete is poured over them. The reinforcing bars are laid out in a rectangular grid of horizontal and vertical bars. At selected intersections between horizontal and vertical bars, they are tied together using a loop of stiff wire which is passed diagonally under the intersection and the two ends of which are twisted together above the intersection in order to tie the two bars together. Traditionally this has been carried out manually although it is difficult, laborious and repetitive.
There are available on the market some powered machines to carry out this task. Another machine is described in WO 2004/083559. Reference should be made to this document for full details although briefly the machine comprises a pair of claws which pass down either side of an intersection between two bars so that a wire can be passed (by means of a guide shuttle) across the gap between the tips of the claws and thereafter drawn up around the intersection and twisted together by means of a rotating spindle.
An improvement demonstrated by the machine disclosed in WO 2004/083559 over prior arrangements is that the wire is allowed to be drawn out of the spindle during twisting by allowing some slippage against the grip on the wire. This helps to prevent the wire breaking under excess tension.
Although the improvements described in WO 2004/083559 should be useful, the Applicant has appreciated that further improvement is possible. One problem which the Applicant has noticed is that it is common practice when in actual use on building sites to use the jaws of machines of the type described generally above, to knock flat the twisted ends of the wire once the twisting operation has been completed in order to prevent things snagging on the twisted wire which may have sharp ends. However, such machines are not designed for this purpose it has been observed that repeated knocks to the lower jaws from such use and other rough handling can quickly cause them to become distorted or misaligned. Since the jaws are essential for guiding the wire in the correct path, any such misalignment or distortion can prevent the machine operating properly.
Another potential problem identified by the Applicant is that achieving the proper tension in the twisted wire relies on the bending strength of the wire and friction exhibited between the wire and the gripping parts of the machine. However, if the surface of the wire or the interior gripping surface inside the machine should be contaminated with oil or grease, or indeed even if the machine is used in damp conditions, the degree of friction actually exhibited may be less than intended leading to a lower tension in the twisted wire and therefore a more loosely tied connection.
The Applicant has further appreciated that an inherent problem with tying machines of the kind described is that since there must always be a region through which the wire passes which is essentially open, to allow the wire to pass around the bars, there might conceivably be an increased risk of injury to an operator or someone else if the wire tying action were initiated accidentally, for example when a part of the body was in the tying zone.
It is an object of the present invention to provide at least some improvement on the aforementioned arrangements.